The electrophotography process used in some imaging devices, such as laser printers and copiers, utilizes electrical potentials between components to control the transfer and placement of toner. These electrical potentials create attractive and repulsive forces that tend to promote the transfer of charged toner to desired areas while ideally preventing transfer of the toner to unwanted areas. For instance, during the process of developing a latent image on a photoconductive surface, charged toner particles may be deposited onto latent image features (e.g., corresponding to text or graphics) on the photoconductive surface having a lower surface potential than the charged particles. At the same time, the charged toner particles may be prevented from transferring or migrating to more highly charged areas (e.g., corresponding to the document background) of the same photoconductive surface. In this manner, imaging devices implementing this process may simultaneously generate images with fine detail while maintaining clean backgrounds.
The precise magnitudes of these electrical potentials and the nature of the voltages (e.g., AC or DC) varies among devices and manufacturers. In general, however, a laser or imaging source is used to illuminate and selectively discharge portions of a photoconductive surface to create a latent image having a lower surface potential than the remaining, undischarged areas of the photoconductive surface. The toner is charged to some intermediate level between the discharge potential of the latent image and the surface potential of the undischarged photoconductive surface. The toner may be charged triboelectrically and/or via biased toner delivery control components, such as a toner adder roll, a doctor blade, and a developer roller. The developer roller supplies toner to develop the latent images on the photoconductive surface. The developed image is ultimately transferred onto a media sheet, typically by employing yet another surface potential that attracts the toner off of the photoconductive surface (or an intermediate transfer surface) and onto the media sheet where it is ultimately fused.
The various surface potentials may be optimized to strike a balance between maintaining clear backgrounds while producing quality images with fine detail. For example, the surface potential of a developer roller may be optimized to develop images with a desired toner density. Another variable termed a “white vector” may be optimized as well. White vector refers to the difference between the surface potential of the developer roller and the surface potential of undischarged portions of a photoconductive surface. An optimal white vector achieves certain desirable characteristics, one of which is to provide a clean media sheet with little or no appreciable background toner in areas other than where printing is desired. Very large white vector values may adversely affect the density of deposited toner and detail of a resulting image. This problem may be more apparent with fine, isolated features where the illumination energy applied to form such features may be insufficient to discharge the photoconductive surface. Conversely, as white vector values fall, unwanted background may begin to appear.
In addition, image quality may be affected by imaging power. Imaging power affects the formation of the latent image on a photoconductive surface. For instance, a low imaging power may be insufficient to discharge the photoconductive surface, particularly with a large white vector. One method of overcoming this problem is to locally control the background energy density on the surface of the photoconductor, particularly in the vicinity of isolated features or isolated clusters of features. The background energy or charge on the photoconductive surface may be controlled on a global basis through some combination of white vector control and discharge via illumination. However, print density variations may call for local control over background energy. As a result, improved image production may be obtained through local modifications of background energy density on the basis of feature density.